Indicator for resistance welding



May 1,. 1956 B. @Ross INDICATOR FOR RESISTANCE WELDING 2 Sheets-Sheet lFiled NGV. 23, 1953 May 1, 1956 B. GRoss 2,744,182

INDICATOR FOR RESISTANCE WELDING Filed Nav. 23, 1953 2 Sheets-Sheet 2Effi J INVENTOR. eoff QJQZWWQE United States Patent O INDICATOR FORRESISTANCE WELDING Bernard Gross, San Diego, Calif., assignol to RohrAircraft Corporation, Chula Vista, Calif., corporation of CaliforniaApplication November 23, 1953, Serial No. 393,787

3 Claims. (Cl. 219-4) This invention relates to resistance welding ofmetallic sheets and to an instrument for visually indicating the severalfactors which co-ntrol the quality of the weld.

ln resistance welding machines, adjustable pressure controls are usuallyprovided for changing the pressure applied to the sheets to be welded,current controls for changing the current passed through the sheets toheat them to the point of fusion at their contacting faces and timingdevices for controlling the sequence of occurrence of these variables.it is diliicult to set these several controls at positions which producethe strongest welds, especially when welding thin aluminum and othersoft metals. it is a main purpose of the present invention to provide aninstrument which may be connected to a spot or other type resistancewelding machine to indicate the changes in pressure and current andtheir sequence of occurrence to each other which take place during thewelding cycle so that the operator can quickly determine whether theseveral controls are properly set or need adjusttnent.

A further object is to indicate the progress of the welding cycle by thetrace of a cathode ray tube and to provide suitable circuit connectionsfrom the welding machine to the control electrodes of the tube whichwill cause the trace to correctly represent the several 'factorsinvolved in the welding cycle in properly timed relation.

A still further object is to provide a standard or comparison which maybe placed over the screen of the cathode ray tube and with which thetrace may be directly compared to determine if the weld represented bythe trace is up to standard.

Further objects of the invention `will become apparent as thedescription of the instrument proceeds. For a better understanding ofthe invention, reference is made to the accompanying drawings in which:

Fig. l is a schematic circuit diagram of an electrical instrumentembodying the invention;

Fig. 2 is a circuit diagram of a pressure pickup used with theinstrument;

Fig. 3 is a schematic View showing portions of a spot welding machineand pressure and current pickups associated therewith;

Fig. 4 is a view partly in section and partly schematic or" portions cfa cathode ray tube and;

Figs. 5 and 6 are views of patterns used as a standard of comparison.

Referring to Fig. l, the electron bezun of a cathode ray tube 1li isvertically deflected by voltage variations imposed on the verticaldeiiecting plates 11 by the leads l1. connected to the output rminals ofvertical ampliiier i3. The input terminals of amplifier i3 are connectedto an electronic switch M which supplies voltage variations frompressure amplilier 15 and current amplilier 16 to amplifier i3.Amplifier 15 is supplied by leads i7 with a variable voltage from apressure pickup 18 to which current is supplied from an A. C. source 19in a manner to be described. Amplifier 16 is supplied by leadsl 20 withla variable voltage from a current pickup ice 21. The horizontaldeections of the electron beam are due to a variable voltage impressedon the horizontal deilecting plates 22 of tube 1li by a pair of leads 23connected to the output of a sweep amplifier 24. Amplifier 24 issupplied with a variable voltage developed in sweep generator 25 underthe control of a trigger device 26. The operation ot trigger 26 iscoordinated with a moving part of the Welding machine 27, portions ofwhich are shown in Fig. 3. The machine has a cam shaft 28 on which cams(not shown) are mounted which control the sequence and timing of thewelding elements in a known manner and also a cam 29 which closes theenergizing circuit 3i) of the trigger when it engages a contact 3i. Anysuitable means may be used for controlling the instant ot closure ofcircuit 30, that shown cornprising a set screw 32 adapted to lock cam 29in any desired angular position on shaft 23. A power source 33 of knowntype supplies current to the several electrodes of tube i0 and the beamis blanked out during its return path by a blanking device 34 of anysuitable known type. Switch 14, sweep generator 25, trigger 26, blankingdevice 34 and the several ampliers contain vacuum tubes connectedtogether in a known manner (not shown).

Figs. 2 and 3 show a preferred method for connecting the pressure andcurrent pickups to the welding machine 27. Lower welding electrode 3S iscarried by copper hom 36 one end of which is secured to a rigid coppersupport 37 fastened to the main frame (not shown) of the weldingmachine. A copper slide 3S mounted for vertical reciprocation in ways 39carries upper electrode All, slide 38 being reciprocated by any knownmeans (not shown) such as a rotating cam or a hydraulic pressurecylinder to press upper sheet 41 against lower sheet 42 with a suitablepressure to eliect a proper weld when the welding current passes throughthe sheets. This pressure is controllable over a substantial range in aknown manner by any suitable means (not shown) and its amount isindicated by trace 43 on the fluorescent screen 44 of tube 1l). Weldingcurrent is supplied by a step-down transformer 45 whose secondarywinding i6 is connected to support 37 and slide 3S. Adjustable currentand timing controls (not shown) of any known type are provided tocontrol the current supplied to primary winding 47.

Current pickup 21 is secured by connecting leads 2l) to points 48 onhorn 36 which are spaced apart a distance sufcient to secure asubstantial voltage drop due to the flow of welding current throughcylindrical horn 36. The ends of the wires may be secured to the horn bysoldering or by suitable clamps (not shown). Or the current pickup couldcomprise a wire coil of many turns wound around either lower electrode35 or upper electrode 4l), the ends of such coil being connected toleads 2i).

Pressure pickup 18 is a strain gage arranged as a Wheatstone bridge andhaving four wires 49, 50, 51, 52 connected together and to currentsource 19 and leads 17 as shown in Fig. 2. Wires Sb and Si are connectedto the top of a metal strip 53 and wires 49 and 52 to the bottom of thestrip, these connections being made by an elastic bonding cement whichalso insulates the wires from the strip. The ends of strip 53 are lirmlysecured to horn 36 by two or more screws 54 which pass through a pair ofmetal spacers 55 which provide suitable clearance between wires 5t), 51and horn 35. As the downward pressure of electrode 4i) progressivelyincreases, the end of horn 36 at the right bends down a small amount andthe right hand end of strip 53 lowers a corresponding amount causingwires Sl), 51 and the cement surrounding them to be put under tensionand wires 49, 52 and the cement around them to be put under compression.With no pressure applied to sheets 41, 42 by electrode 4l),`

the bridge is balanced and its conjugate points A and B are at" thesainel ptn'ti'al. As the pressure on electrode 4G'increases, itcauseshorn 36 to bend increasingly and cause Wires 50, 51 to be placedunder tension and elongated with a corresponding increase in theirresistance. This causes a potential difference in a certain directionbetween points A and B. This potential diiierence is increased by thesimultaneous decrease in the resistance of Wires 49 and 52. It followsthat the voltage across the points A, B varies with the pressure appliedto sheets 41, 42 by the welding electrodes and this pressure changeduring the weld causes a corresponding vertical displacement of theelectron beam. The trace d3 shown in Fig. 5 is typical of the pressuressecured during a welding cycle when welding aluminum alloy sheets about1/16 inch thick. Starting at the left the trace indicates an abruptincrease in pressure to establish a forming pressure indicated at 56 ofsubstantial duration after which a rapid drop in pressure takes place toa welding pressure 57 which is substantially steady for a short time.While the welding pressure is applied, a large welding current flowsbetween electrodes 35, iti which fuses sheets fil, 42 at theircontacting faces to form a small weld nugget. This sudden liow ofwelding current causes a potential drop between points 48 (Fig. 3) whichis indicated on the screen by an abrupt rise 53 in the welding currenttrace 59. Shortly after the welding current has stopped, the electrodepressure increases again to a forging pressure indicated at di) whichpersists while the welding nugget is cooling and soliditying. After thisthe welding pressure decreases as upper electrode di) is raised until itreaches zero when the Welding cycle is complete. Cam 29 is preferablyset at a position on shaft .7.8 which initiates the action of trigger 26so that the horizontal sweep of the beam is not commenced until upperwelding elec trode 40 touches sheet 4l. The trigger 26 and amplifier24.- then continue sweeping the beam in one direction during the weldingcycle, the beam being blanlred out while on its return trace by blankingcontrol This arrangement prevents beam traces from appearing on screen4d when no welding is going on. Assuming the welding cycle described toproduce a weld of adequate strength to hold the sheets 4l., 12 together,a full size photograph is then taken on iilm or the end ot tube it)during a Welding operation by supporting a camera in line with the ytubeaxis. After linishing the photographic negative and trimming it to theshape and size of the end of the tube, it will contain exactreproductions of pressure trace 43 and current trace S9 on transparentfilm. This negative dit is now attached in any suitable manner to theend of tube as by cementing, clamping or other known means. The pressureand current graphs on negative 6ft serve as a standard of comparisonwith which the operator may compare the traces obtained on the screen d4as each weld is made to determine if the current and pressure controlsot the welding machine are properly adjusted. For example, it a currenttrace indicated by dash lines at 62 is secured on screen rid during aweld, by comparing it with the standard current peak 58 on tilm 6l, theoperator will know that the weld does not have the proper strength. Hewill also realize that the controls of the machine which control thetiming and amplitude of the welding current are not properly set toproduce the best weld and need adjusting. The beam trace on screen 44may be observed as the adjustments are made and when it matches thegraph or pattern on film tilt, serves as an indication that the controlsare properly set to produce the best weld.

Instead of securing line line pressure and current traces by havingtheelectron beam well focused on screen de., these may be widened bymisocusing it by an amount sutiicient that the edges of the tracesindicate the positive and negative tolerances of the variables. Byphotographing screen 44 including such wide traces and melting apositive of the finega'tive'on flexible iilm, a positive 53 of the typelshownlin Figure-dis secured. ThebandsV 64 and 65 are transparent andsurrounded by the black area of theilm. When welding; if the entirepressure trace on screen i4 is visible to the operator through band 6d,it indicates that the several pressures above mentioned come within theprescribed tolerances. Also if the entire trace 59 of the weldingcurrent is visible through transparent band 65, it indicates that thewelding current is properly timed and within the prescribed amplitudetolerance.

instead of attaching the strain gages to strip 53, the wires Si?, may becemented to the top of horn 36 and the wires e9, 5?, cemented to thebottom thereof. The increase in resistance of wires 5l), 51 and thedecrease in resistance or" wires 49', 52 due to the liexure of horn 36contribute to the unbalancing of the bridge as before.

This invention may be embodied in other forms or carried out in otherways without departing from the spirit or essential characteristicsthereof. The present embodiment of the invention is therefore to beconsidered as in all respects illustrative and not restrictive, thescope of the invention being indicated by the appended claims, and allchanges which come within the meaning and range of equivalency of theclaims are intended to be embraced therein.

Having thus described my invention, what l claim as new and useful anddesire to secure by Letters Patent is:

l. h/ieans for pressing and welding together upper and lower contactingmetal sheets and for simultaneously indicating the variable pressureapplied to the sheets comprising, in combination: a stationary member; along cantilever support having one end anchored to said member, saidcantilever support being constructed to bend under said variablepressure and having high electric conductivity; a lower weldingelectrode in contact with and secured to the free end of said cantileversupport, the top of said lower electrode being in contact with thebottom face of said lower sheet; an upper welding electrode in contactwith the top face of said upper sheet and supported for downwardmovement sutiicient to press said upper sheet against said lower sheetand simultaneously bend said cantilever support; a source of weldingcurrent connected to said cantilever support and to said upperelectrode; a resistor; means connecting said resistor to said cantileversupport and constructed to insulate said resistor therefrom and tochange the physical dimensions and resistance of said resistor inresponse to the bending of said cantilever support; a current sourceconnected to supply current to said resistor; an amplifier connected tothe terminals of said resistor for amplifying the voltage variationsbetween said terminals; and visual indicating means connected to theoutput of said amplifier and constructed to indicate the instantaneousamplitudes of said amplified voltage variations.

2. Means for indicating the variable pressure applied to two contactingmetal sheets by a pair of welding electrodes in engagement with thesheets comprising, in cornbination: a cantilever support in contact withone of said electrodes and supporting said electrode on the free end ofsaid support, said support being bent by pressure of the sheets on saidone electrode; a normally balanced Wheatstone bridge comprising fourstrain gages; means supporting said bridge on said cantilever in amanner to increase the resistance of two oppositely disposed straingages and to simultaneously decrease the resistance of the remaining twostrain gages in response to the bending of saidcantilever; a source ofvoltage connected across one diagonal of said bridge; an oscilloscopehaving horizontal and vertical deliecting plates; an amplifier havingits input connected across the other diagonal of said bridge; a circuitconnecting the output of said amplifier to said vertical detlectingplates; and a sweep generator connected to said horizontal deflectingplateswhereby the beam trace represents the variable welding pressure.

3. Means for pressing and welding together upper and lowercontactingmetal sheetslv and for simultaneously indicating the variablepressure applied to the sheets cornprising, in combination: a stationarymember; a long cantilever having one end anchored to said member, saidcantilever being constructed to bend under said variable pressure andhaving high electric conductivity; a lower welding electrode in contactwith and secured to the free end of said cantilever, the top of saidlower electrode being in contact with the bottom face of said lowersheet; an upper welding electrode in contact with the top face of theupper sheet and supported for downward movement sufiicient to press saidupper sheet against said lower sheet and simultaneously bend saidcantilever; a `source of welding current connected to said cantileverand to said upper electrode; a resistor having first and second portionsconnected to a common intermediate terminal; a current source connectedto said resistor; means for supporting said resistor by said cantileverand for insulating said resistor from the cantilever, said resistorsupporting means being constructed to tension said first resistorportion and to simultaneously compress said second resistor portion inresponse to the bending of said cantilever under the pressure of saidupper electrode against the upper sheet; an amplifier having its inputterminals connected to the ends of said resistor; an oscilloscope havinghorizontal and vertical deflecting plates; a circuit connecting saidvertical deecting plates to the output of said amplifier; and a sweepcircuit connected to said horizontal deliecting plates to provide asweep control of the electron beam simultaneously with its deection bysaid vertical deflection plates.

References Cited in the file of this patent UNITED STATES PATENTS2,322,052 Ogden et al June 15, 1943 2,430,237 Moncher Nov. 4, 19472,449,093 Weingarten Sept. 14, 1948 2,616,014 Ellerby Oct. 28, 19522,657,133 Weingarten et al. Oct. 27, 1953

